Receiving apparatus for communication systems



May 7, 194.6- l.. D. WHITELocK 2,399,965@

RECEIVING APPARATUS FOR COMMUNICATION SYSTEMS Filed April A7, 1942 iM5001 55904:- V

f fcgv C0 Vllla e Gaz?? Patented May 7, 1946 RECEIVING APPARATUS FORCOMIVIUNICATION SYSTEMS Leland D. Whitelock, Bethesda, Md., assignor toThe vUnion .Switch and Signal Company, .Swiss- ,vale, Pa., a corporationof Pennsylvania Application April 7, 1942, Serial No. 437,966

2 Claims.

My invention relates to receiving apparatus for communication systems,and more particularly to receiving apparatus which includes an electrontube amplifier.

Communication systems using electron tube amplifiers in the receivingapparatus .frequent' ly require automatic volume control .means tocontrol the amplifier .gain .during periods in which the energy level ofthe input current exceeds a normal value.

A feature of my invention is the provision oi a communication systemreceiving apparatus incorporating novel andV improved vmeans forcontrolling the amplier gain of electron tubes used in the receivingapparatus.

Another feature of my invention is the provision of receiving apparatusoi" the type here involved including a novel automatic volume controlmeans provided With a compensator characterized by a falling resistancewhen the voltage applied thereto increases and by which compensator thenon-lineal` characteristic of the gain curve of an electron tube iscompensated.

Other features, objects andradvantages of my invention will appear 4asthe specication progresses.

.I shall describe two forms of apparatus ,ernbodying my invention andshall .then point .out the novel :features thereof in claims.

In the accompanying drawing, Fig. 1 is a diagrammatic view showing .oneform of receiving apparatus embodying my .invention for use ,in acommunication system employing ,an oscillating current, `such as.acarrier .telephone current. Fig. 2 is a diagrammatic view showingapparatus embodyngmy invention suitable for use for radio reception.Figs. 3, 4 and5 are diagrams of operating characteristics of theapparatus of Figs. 1 and '2. In each of the different viewslikereference characters are used to designate similar parts.

Receiving apparatus of the type here contemplated frequently employs arectifier to rectify a portion of the loutput of `a final -orapreselected stage -of the receiving apparatus for Aobtaining .lavariable direct -voltage which used to control the .amplifier gain ofone or :more earlier lstagcs of the apparatus. The sa.iT-ura-tion of theavailable .automatic volume control voltage obtained by such-alcircuitv-is illustrated .by-,curve A .of Fig. 3. The curve .of Fig. 4illustrates the change 4in voltage gain of an ampliner `tube with achange in grid bias. It .is to be seen ,from the .curve of Fig. 4, thatthe desired type of curve 'for automatic volume control voltage Where asubstantially constant energy level of the output ilrrcnt is to `bemaintained .is that .illustrated by curve B oi Fig. 3. A comparison of.curves A and i3 of Fis. 3 readily shows why a substantially con-V stantoutput .current may not be maintained by the circuit arrangement.heretofore usedlfor obtaining an automatic volume contro1 voltage. To,improve the automatic volume control means heretofore used, I intarposea compensator having a falling resistance characteristic of the natureillustrated 'by the Vcurve of Fig. 5 the automatic volume control meansandby the use of which compensator the non-linearity of the amplifiergain curve of an .electron tube Vis ccmpensated with the result that anautomatic volume control voltage of the .character illustrated by curveB of Fis. 3 is obtained.

Referring now to Fig. 1A, the receiving apparatus includes a rst .stage.of high frequency amplification, a demodulator-lter and one stage oflow or .audio frcoucncy amplication. The rst .or 'high frcquenoystagc ofamplcation includes an .electron tube 2 which as here shown is anindirectly heated cathode tube, the filament of which is constantlyheated in .the usuel manner. `A receiving .or input circuit 'indicatedby the reference character IC is .supplied at some remote stationnctvshown with an oscillating current.. .such as. .for example, .a`.carrier telephone current, and is coupled .to the controlsrd 5 andcathode of tube 2 through atransforrner TI, the control grfldcircuit oftube 2 'being completed through a biasing unit BUI and ground electrodes1 and 8- 'Plate Circuit voltage vis applied to plate 9 and cathode `ti of tube'Z from` a source Whose terminals are indicated at `Bitland N300,and which plate circuit includes 4winding 110 of a coupling transformerT3 -to couple the plate circuit of tubel with -a demodulator-lter DG,windings -IE and l1 of transformer T3 `being connected tothe input ofdemodulator-filter DO. Demodulator-filter shown conventionally since.its specic structure forms no A:part .of .my invention. It may 'betof,anylone of sev-eral wellknown .forma ,for example, it may be Va.balanced demodulator with a band passilter. .Itis suincent fior the:present :application to point `out that a modulated 4,canin .current.such .as a sinele vside band telephone current .is mixed with a carrierfor demodulation and vthon .applied ,to a band pass .nltcrso that ,the.voice frequencies .only are .supplied to winding i8 .of a transformerT4. Winding I8 of transformer Til 'is in turn connected across controlgrid I9 and cathode 20 ofanelectron tube 4 of the audio or low frequencystage of amplification. Tube 4 as here shown is of the indirectly heatedcathode type, the filament of which is constantly heated and the plate22 of which is supplied with voltage from the E300-N300 current sourcethrough a winding 2| of an output transformer T5. A secondary winding 23of transformer T5 is connected to a loud speaker LS so that theY voicefrequenciesV supplied to tube 4 are reproduced at the loud speaker.

An automatic volume control means is connected across the output circuitof tube 4 to be energized by a portion of the alternating currentcomponent of the plate circuit current.

This means comprises a condenser 24, a full wavel rectifier 25, acontrol unit CU consisting of a resistor 21 and a condenser 28 inmultiple, an-

other condenser 26 and a compensator CO. The positive output terminal ofrectifier is connected to one terminal of unit CU through thecompensator CO, and the same terminal of unit CU is also connected to aground electrode 30. The other terminal of unit CU and the negativeterminal of rectifier 25 are connected to the control grid 5 of tube 2through a connection in-' cluding wire 3l and a resistor 34.V A portionof the alternating current component of the plate circuit current fortube 4 is rectified by rectifier 25 and the resulting unidirectionalcurrent is caused to flow through compensator CO and resistor 21 of unitCU so that condenser 28 is charged at a potential substantially equal tothe average voltage drop across resistor 21, the flow of current beingin such direction that the positive terminal of unit CU is adjacent theground electrode 30. Compensator CO isv characterized by a fallingresistance characteristic. For example, compensator CO may be a copperoxide rectifier element of the type having a falling resistancecharacteristic of the form illustrated by the curve of Fig. 5. That isto say, such an element has in its low resistance or pass direction arelatively high resistance when low voltagesare applied thereto and suchresistance falls rapidly in value to a relatively low value as thevoltage applied thereto is increased. Compensator CO is interposedbetween the positive output terminal of rectifier 25 and thecorresponding terminal of unit CU, and the parts are soA proportionedthat a normal predetermined output energy level for the plate circuit oftube 4 impresses a relatively loW voltage across compensator CO and unitCU with the result that the resistance of compensator CO is relativelyhigh and is large as compared with the fixed resistance of resistor 21.This means that the major portion of such direct voltage is consumed ina voltage drop through compensator CO. When the energy level of theplate circuit of tube 4 is relatively large, the voltage impressedacross compensator CO and unit CU is increased so that the resistance ofcompensator C0 is noW relatively low while that of resistor 21 issubstantially the same as before. Under this condition the major portionof the direct voltage is consumed by resistor 21, and only a smallportion is consumed at compensator CO. Looking at Fig. 5, thecompensator CO is arranged to be Worked at a point such as X of thecurve at'the normal output of the amplifier, and is worked at anotherpoint such as Y, of its operating curve for an output of largemagnitude. The voltage developed across unit CU is applied to thecontrol grid of tube 2 as an automaticA volume control voltage, thisvoltage being ap-.

plied to the control grid 5 of tube 2 through the connection includingresistor '34.

It is to be seen therefore that with the apparatus of Fig. 1 an incomingcarrier telephone current of a normal predetermined value is amplifiedat tube 2, demodulated, further amplied at tube 4 and reproduced at theloud speaker, theoutput energy level of the amplifier tube 4 beingsuchthat substantially noY bias voltage is applied through unit CU to thetube 2. As the magnitude of such incoming current increases, the chargecreated across unit CU is built upl along a curve corresponding to curveB of Fig. 3 due to the characteristic of compensator CO. That is, theautomatic volume control voltage created across unit CU increases fasterthanthe energy level of the output of the amplifier.. Such automaticvolume control voltage' applied to tube 2 reduces the amplier gain'sothat a substantially constant value of output current for the amplifieris obtained. Additional stages of amplification canv be of courseincluded in the receiving apparatus if desired and the automatic volumecontrol voltage derived through unit CU applied to such additionalstages if desired.

Referring to Fig.V 2, I have here shown the essential parts of a radioreceiver apparatus. An input circuit IC connected to an antenna orsimilar device is coupled to the control grid of first stage tube 2through transformer TI, while the output or plate circuit of tube 2 iscoupled through a transformer T6 to a detector shown conventionally atDE. The specific construction of such detector DE is immaterial and maybe any one of several well-known arrangements. The output of detector DEis connected across control grid I9 and cathode 20 of tube 4 of afollowing or intermediate stage of amplification. The plate circuit oftube 4 includes a primary Winding 4l] of a, transformer T1. Onesecondary winding 4| of transformer T1 is connected to an output circuitnot shown, and another secondary winding 42 connected to an automaticvolume control means which'includes a twin diode rectifier tube BHG, acontrol unit CU, a filter comprising a resistor R2 and a condenser CZ,and a compensator CO. The portion of the amplified energy passed towinding 42 of transformer T1 is rectified at anode 43 and cathode 44 oftube GHG and supplied to unit CU, the resistor 21 of unit CU acting as aload resistance and condenser 28 of the unit being charged at apotential substantially equal to the average voltage drop acrossresistor 21, the same as in Fig. 1. A battery 41 provides a xed voltagewhich is applied to anode 45 and cathode 46 of tube GHB through resistor21 of unit CU, resistor R2 and compensator CO, compensator CO beingcharacterized by a falling resistance the same as in Fig. 1. At thenormal voltage of battery`41, compensator CO is operated at point Y ofits characteristic curve as illustrated in Fig. 5. When theelectromotive force of Winding 42i of outputtransformer T1 becomeslarge; the

rectified current fiowlng in resistor 21 creates a voltage drop thattends to cause anode 45 to be less positive in potential with respect tocathodeV 46. This change tends toA cause a lower voltage to be appliedto compensator "CO andl hence its resistance rapidly increases with the'result that the automatic volume control. voltage applied tol controlgrid 5 of tube 2 through a' connection including resistor 48 andwire49finfcreases in a manner illustrated by curve B of Fig. 3.

It is to be pointed out that the compensator CO of Fig. 2 may be'interposed in the connection between winding 42 and anode 43 of tubeSHG, in which case it will govern the automatic volume control voltageby governing the voltage drop across unit CU.

Apparatus embodying my invention has the advantage that as the energylevel of the input current increases above a predetermined normal value,the automatic volume control voltage is increased at a faster rate andso the non-linear condition of the amplifier curve of an electron tubeis compensated. Also, apparatus embodying my invention tends to increasethe initial sensitivity of the automatic volume control circuit andtends to maintain a highly uniform output energy level.

Although I have herein shown and described but two forms of receivingapparatus for communication systems embodying my invention, it isunderstood that various changes and modifications may be made thereinwithin the scope of the appended claims withoutv departing from thespirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In receiving apparatus for use with an oscillating communicationcurrent, the combination comprising, a plurality of electron tubeamplier stages arranged in cascade, a grid circuit for the tube of afirst one of said stages to receive such communication current, a platecircuit including a source of direct current for the tube of a final oneof said stages, a multiple Winding transformer having a first windinginterposed in said plate circuit, a receiver connected to a secondwinding of said transformer to receive said communication current asamplified by said stages, a twin diode rectifier tube, a control unitconsisting of a first resistor and a condenser in multiple, acompensator characterised by a falling resistance as the applied'voltageincreases, a. battery, a second resistor; said control unit connected tosaid battery through said second resistor, said compensator and one sideof said rectifier tube in series to create a normal voltage drop acrosssaid unit; said unit connected to a third winding of said transformerthrough the other side of said rectifier tube to create across said unita. direct voltage proportional to the amplitude of said communicationcurrent, and said control unit and said second resistor in seriesconnected between the grid and cathode of the tube of at least one ofsaid stages to control the amplifier gain of that tube by the resultantvoltage thus created across said control unit.

2. In receiving apparatus for use with an oscillating communicationcurrent, the combination comprising, a plurality of electron tubeamplier stages arranged in cascade, a grid circuit for the tube of afirst one of said stages to receive such communication current, a platecircuit including a source of direct current for the tube of a final oneof said stages to reproduce said communication current as amplified bysaid stages, an output circuit coupled to said plate circuit to receivesuch amplified communication current, a control unit consisting of aresistor and a condenser in multiple, a rectifier, a compensatorcharacterized by a falling resistance as the applied voltage isincreased; circuit means including said control unit, rectifier andcompensator coupled to said plate` circuit to receive such communicationcurrent in multiple with said output circuit to create a voltage acrosssaid unit that is amplified proportional to the communication currentand is made to vary at a rate that substantially corresponds to theamplifier gain curve of the tube of said first stage, and other circuitmeans to connect said control unit to the grid circuit of said firststage tube to govern its gain by the voltage thus created across saidunit.

LELAND D. WHI'I'ELOCK.

